For the first challenge, I’d go one small step further and ask theists to define what they mean by a god, as opposed to the particular god that they worship. That is, if you met a god on the street, how would you know it? And how would you know whether it’s Yahweh or not?

The reason this is a challenge rather than a big duh is that “God” is a word that refers to a concept that varies widely from person to person. The only thing that (mono)theists have in common is that whatever it is that they believe in, they all call it “God”. If “God” referred to something real outside of people’s heads, you’d think there would be some consensus as to what the word means, or at least as much consensus as for the word “art”.

The second challenge, that of proving one’s faith the way Abraham did, brings up an interesting point: that the god of Abraham isn’t above accepting human sacrifice.

Assume for the sake of argument that you’re a Christian parent. If you heard a voice claiming to be God and telling you to sacrifice your child, presumably you would think, “God wouldn’t ask me to do something like that. This must be a trick” (whether this “trick” is a hallucination, or pranksters in the bushes, or demons, is not important now). And yet according to the story, Abraham immediately went to sacrifice his son Isaac, not knowing that an angel would come to stay his hand at the last moment.

In other words, as far as Abraham knew, it was entirely plausible that Yahweh would ask him to kill his own son. And this was a guy who had talked to Yahweh regularly, so he should know.

(There’s also evidence that in an earlier version, Abraham did sacrifice his son, and that the angel was added later, when people had moved away from human sacrifice. But that’s not important here.)